1. Field of Invention
This invention relates to increasing the oxidation resistance of molybdenum, its preparation and its use in electric lamps for seals between molybdenum and a vitreous material. More particularly, this invention relates to a method for increasing the oxidation resistance of molybdenum exposed to an oxidizing environment at temperatures between about 250.degree.-600.degree. C. and its use for increasing the life of hermetic seals between molybdenum and electric lamps employing such seals, wherein that portion of the molybdenum in the seal area exposed to the oxidizing environment is coated with alkali metal silicate.
2. Background of the Disclosure
The use of molybdenum foil for effecting a hermetic seal with vitreous materials, such as pinch seals and vacuum-formed seals for quartz lamp envelopes, is old and well known to those skilled in the art. Molybdenum is an oxidation-sensitive material and oxidizes rapidly in an oxidizing environment such as air at temperatures of about 350.degree. C. and higher. In the case of molybdenum foil used for hermetic pinch and vacuum-formed seals, this oxidation can result in an open circuit or can crack open the seal, either of which results in lamp failure. In many instances it is also preferred to use molybdenum wire for the outer current conductors which should be locked deeply in the seal area, so as to be able to withstand the forces which occur when the lamp is connected to the current source. Most quartz-molybdenum hermetic seals are satisfactory up to a seal temperature of approximately 350.degree. C. At temperatures of about 350.degree. C. and higher, the rate of the oxidation reaction between the oxygen in the surrounding atmosphere and the molybdenum foil greatly increases and results in a substantial reduction in the useful life of lamps employing hermetic seals between molybdenum and a vitreous material. The oxidation reaction takes place, because during the sealing operation microscopic passage ways are formed around the lead wires as the vitreous material cools. The passage ways or cracks permit oxygen to enter the foil area of the lamp seal.
In the formation of a pinch seal or vacuum seal with a vitreous material such as quartz, the quartz does not completely attach itself to the relatively heavier outer and inner lead wires, due at least in part to the relatively high viscosity of the quartz. Another reason for the microscopic passageways, which exist not only along the outer lead wire, but also along the outer edge of the foliated portion perpendicular to the transverse axis of the lamp, is the substantial difference in the coefficient of thermal expansion of the quartz compared to that of the refractory metal outer lead wire, which is usually tungsten or molybdenum.
These seals have always been a source of potential premature lamp failure and many attempts have been made to produce better seals. Efforts have been made in the past to prevent the oxidation of that portion of the molybdenum foil area which is exposed to atmospheric oxygen because of the passageways formed in the pinch seal. One such attempt, in U.S. Pat. No. 3,420,944, discloses coating the outer half of the molybdenum foil with a thin film of chromium. This was accomplished by forming the foil seal from two pieces of molybdenum foil. One piece was plated with chromium and the other was not plated. Both pieces were then tack welded together. Although this solved some oxidation problems, it created other problems relating to reduced mechanical strength and foil flatness. Also, if the chromium coating was too thick it introduced an oxygen passage from the outside of the lamp envelope to the unplated foil portion. Consequently, another attempt was made which is disclosed in U.S. Pat. No. 3,793,615. This patent discloses a tungsten-halogen lamp having a pinch seal over molybdenum foil wherein only about half of the molybdenum foil is coated with a layer of chromium. The plating is in the form of a wedge or taper, with the greatest thickness of the chromium layer being at the outer edge of the foliated portion and a comparatively thin portion located on that part of the foil which forms a part of the hermetic seal between the foil and quartz. This patent also suggests that the chromium film may possibly be replaced by nickel, molybdenum disilicide and alloys of chromium and nickel.
In U.S. Pat. No. 4,015,165 a proposed solution to a problem of the oxidation of molybdenum outer current conductors of electric lamps having a quartz glass lamp envelope with a pinch seal consists of covering the molybdenum outer conductors with a coating or sleeve of oxidation resistant material, such as nickel plated brass. U.S. Pat. No. 4,539,509 discloses applying a sealing glass composition to the small space or passage between the outer leads and the quartz. The sealing glass becomes molten at temperatures above 350.degree. C. and thereby forms a hermetic seal between the quartz and conductors.
More recent attempts to alleviate the oxidation problem of molybdenum foil seals exposed to air are disclosed in U.S. Pat. Nos. 4,677,338 and 4,682,071 which relate to both incandescent lamps and discharge lamps having quartz envelopes with substantially elongated stem portions for the pinch seal. The outer face or surface of the elongated seal area stem is highly polished or coated, ribbed, twisted or otherwise modified so that a portion of radiation incident upon it from the light source is directed away from the foil and adjacent region of the terminal conductor. This is done in order to reduce the temperature of the seal area at the outer portion and thereby reduce oxidation of the molybdenum. The '358 patent also states that lamp failure due to oxidation of the molybdenum can be a problem at temperatures as low as about 250.degree. C.
Notwithstanding the above, a serious problem still exists with respect to preventing the oxidation of both molybdenum foil seals at the foil-air interface and molybdenum or molybdenum coated conductors or other objects exposed to an oxidizing environment at temperatures above about 350.degree. C. Thus the need still exists for a practical and facile solution to the problem of such molybdenum oxidation.